So do you think Kim forgave Nick in the end?

No. SPOILER ALERT! Prom Night is a horror movie not only because of the murders that take place but also for what happens to its main character--Kim. The popular prom queen's entire world is turned upside down. Not only was her sister killed, but she was killed by her best friends and boyfriend. To make matters worse, her best friends were then murdered by her brother, who also attempted to murder her boyfriend. To top it off, she inadvertently kills her own brother with whom she was very close. Then you add the fact that her family's reputation is ruined; after all, when the principal's son goes on a killing spree at the very school entrusted to the father's care, that cannot be good. And Kim will always remember this because it happened...on Prom Night! The point is Kim ends up so screwed up, there's no way she can continue a relationship with Nick.

Let's not forget that Kim's mother Mrs. Hammond was already unhinged by Robin's death. After the prom, my guess is the poor lady took a one way trip to the mental hospital.

Not after learning of him being involved with her losing both siblings.